Supercritical fluids are those fluids which exist as fluids at a temperature and pressure above the fluid critical temperature, T.sub.c, and fluid critical pressure, P.sub.c, respectively. As used herein, the term supercritical fluid may be either a pure fluid or a mixture of two or more fluids. The fluid critical temperature, T.sub.c, and the fluid critical pressure, P.sub.c, for most substances, including common liquids, are known and are defined, respectively, as that temperature and pressure above which a liquid and its vapor cannot coexist in equilibrium. However, in a system maintained in such that both the system pressure and temperature are higher than the critical temperature and pressure of the compound or mixture of compounds contained in the system, the system is said to contain a fluid under supercritical conditions. Supercritical fluids exhibit a number of advantageous properties when compared with conventional liquids, in particular, supercritical fluids may have improved properties as solvents. For example, the diffusivity of many substances may be greatly enhanced in a supercritical fluid as opposed to a conventional liquid of comparable density. The viscosity of a supercritical fluid may also be much lower than a conventional liquid solvent of comparable density. These two properties may make a supercritical fluid advantageous in facilitating mass-transfer operations. Another property of a supercritical fluid may be its sensitivity to the solubility of a particular compound therein to small changes in temperature and pressure. As an example, the solubility of naphthalene in supercritical carbon dioxide may change by a hundred-fold by small changes in temperature and pressure of the supercritical fluid. This sensitivity of solubility to small changes in temperature and pressure of a supercritical fluid has been used to effect separation of products by their solubilities. See, for example, U.S. Pat. No. 3,969,196. Separation schemes based on changes of solubility by small temperature and pressure perturbations of supercritical fluids have been used for extraction of caffeine from coffee beans (C. H. Kurzals, "Caffeine Extraction," presented at the Soc. Chem. Ind. Food Eng. Panel Symposium, CO.sub.2 in Solvent Extraction, London, (1982)), and for extraction of hops for beer production (See D. S. J. Gardner, "Industrial Scale Hops Extraction," presented at the Soc. Chem. Ind. Food Eng. Panel Symposium, CO.sub.2 in Solvent Extraction, London (1982)).
Another feature of supercritical fluids is that by selecting a supercritical solvent with a proper T.sub.c, an extraction process may be conducted at relatively low temperatures, thus avoiding potential denaturation or decomposition of heat-liable compounds. Low temperature supercritical fluid processing has been utilized for extraction processes. However, application to other processes has been thus far reported only in the case of the hydrolysis of a biomass using supercritical water. See Vick Roy, J. R., and Converse, A. D., "Biomass Hydrolysis With Sulfur Dioxide and Water in the Region of the Critical Point", presented at the 1984 AICHE Annual Meeting, San Francisco, November 1984.
With respect to enzyme-catalyzed reactions in nonaqueous solvents, there has been work reported by Zaks, A., and Klibanov, A. M., Science 224 1249-1251 (1984), and Butler, L. G., Enzyme Microb. Technol. 1, 253-259 (1979). However, this work involves use of conventional liquid solvents.